1. Field of Invention
This invention relates in general to devices for tensioning risers for offshore well production, and in particular to a device that causes the upper portion of the riser to rotate along with the vessel if the vessel rotates due to waves and currents.
2. Description of Related Art
Offshore production platforms must support production risers from oil or gas wells which extend to the platform from subsea wells. For platforms that are fixed to the ocean floor this is readily accomplished and is well known in the art. However, for subsea completions in deep water that require the use of floating platforms, such as tension leg platforms or semi-submersible platforms, supporting risers present significant problems. These platforms move under the influence of waves, wind, and current and are subjected to various forces, including rotational forces. Thus, the riser tensioning mechanism must permit the platform to move relative to the riser and care must be taken that the tensioning mechanism can withstand rotational forces.
The riser tensioning mechanism must also maintain the riser in tension so that the entire weight of the riser is not transferred to the wellhead and so that the riser does not collapse under its own weight. The tensioning mechanism must therefore exert a continuous tensional force on the riser. Also, this force must be maintained within a narrow tolerance. The use of a hydraulic cylinders attached between a platform and a riser to support the weight of the riser is well known in the art. The tensioner cylinders of the prior art are exposed to torque resulting from vessel rotation in response to environmental forces such as waves, currents, and wind. Improvements to protect the tensioner cylinder from this torque are desired.